Multi-mull phase control system

ABSTRACT

A multiple stage mixer including a pair of mixing chambers in communication with each other for the transfer of material therebetween and a rotary mixing element in each chamber movable on a path overlapping into the adjacent chamber in selected normal phase relation with respect to the mixing element therein. A source of motive power for rotatively driving said mixing elements and control means for rendering said motive power source ineffective when said mixing elements are out of said normal phase relation with each other is provided in order to prevent damage to the mixer in the event the phase relation between the mixing elements becomes abnormal.

[ 1 Oct. 30, 1973 Elte States Patent 11 1 Troy [54] MULTI-MULL PHASEONTROL SYSTEM 3,334,822 8/1967 Mcllvaine 241/124 [75] Inventor: ElbertC. Troy, Highland Park, 111.

[73] National Engineering Company,

Chicago, 111.

Primary Examiner-Robert W. Jenkins Attorney-Richard D. Mason et al.

Assignee:

[57] ABSTRACT A multiple stage mixer including a pair of mixing chambersin communication with each other for the transfer of materialtherebetween and a rotary mixing 22 Filed: June 26,1972

21 Appl.No.:266,430

element in each chamber movable on a path overlapping into the adjacentcham phase relation with respect therein. A source of motive power ingsaid mixing elements and control means for rendering said motive powersource ineffective when said 0 6 M21 2 954 5 0 5 m n. 2, B 3 .H3 2 "W2 1l u m u 4 4 2 MHZ 9 NW9 7 ""11 1 1. w "U,

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kwwmm MULTI-MULL PHASE CONTROL SYSTEM The present invention relates to anew and improved multiple stage mixing apparatus and is an improvementand refinement of the apparatus shown, described and claimed in US. Pat.No. RE 25,475 dated Nov. 5, 1963. In multiple stage mixing apparatus ofthe type shown in the foregoing patent there are employed a pair ofrotary mixing elements, which elements, pass over a common area at thejunction of the respective mixing chambers. Each mixing element isdriven to rotate in a selected phase relationship with respect to theother element in order to effect a flow or transfer of material from theinlet or charging chamber to the outlet or discharging chamber in acontinuous mixing process. If one of the rotary mixing elements for anyreason fails to rotate when power is being supplied, or for any reasonthe selected phase relation between the two mixing elements becomesabnormal there is a chance of direct collision and engagement betweenthe mixing elements in the common area at the junction of the respectivechamber. For example, should a foreign object become lodged in one ofthe mixing chambers and because of excessive torque cause a shear pin inthe drive train of the mixing element to fail, the mixing element wouldstop rotating while the mixing element in the other chamber wouldcontinue to rotate at normal speed. If the disabled mixing elementhappens to stop with its outer plow in the common area at the junctionof the mixing chambers, the outer plow of the operative mixing elementwould then collide directly with the stopped element. After maintenanceon the power train of the mixer, the phase relation between the pair ofmixing elements may not be properly reestablished and operating themixer in this condition can cause extensive damage to the mixingelements if a collision between the plows occurs.

The present invention provides a new and improved multiple stage mixingapparatus of the general type described wherein means is provided forsensing the phase relation between the mixing elements rotating in theseparate mixing chambers and for shutting down the source of rotativepower when an improper phase relation condition is sensed.

The foregoing is a primary object and advantage of the present inventionand another object is to provide a control system for monitoring thephase relation between the mixing elements in a multiple element mixingapparatus of the character described.

Still another object of the present invention is to provide a new andimproved multiple stage, continuous mixer having means for automaticallydisconnecting the rotative power source for the mixing elements wheneverthe mixing elements are out of the selected phase relation relative toone another.

Another object of the present invention is to provide a new and improvedcontinuous mixing system wherein the equipment used to feed materialinto a continuous mixer is automatically stopped upon sensing acondition of improper phase relation between the rotary mixing elementsof the mixer.

Yet another object of the present invention is to provide a new andimproved mixer control system adapted to shut down the mixer in theevent of a sheared drive pin on one of the mixing elements, or in theevent of an out of phase relation between the separate mixing elements.

The foregoing and other objects and advantages of the present inventionare accomplished in an illustrated embodiment comprising a new andimproved multiple stage mixing apparatus having a pair of mixingchambers in communication with each other for the transfer of materialstherebetween and a rotary mixing element mounted in each chamber movableon a path overlapping into the other chamber and in selected normalphase relation with respect to the mixing element therein. The mixerincludes a source of motive power for rotatingly driving the mixingelements and control means is provided for rendering said motive powersource ineffective any time the elements are sensed to be out of saidnormal phase relation with respect to one another.

For a better understanding of the present invention reference should behad to the following detailed description when taken in conjunction withthe drawings in which:

FIG. 1 is a top plan view of a multiple stage mixer constructed inaccordance with the present invention;

FIG. 2 is a fragmentary, vertical sectional view taken substantiallyalong lines 22 of FIG. 1; the

FIG. 3 is a schematic diagram illustrating the new and improvedcontinuous mixing system in accordance with te present invention;

FIG. 4 is a fragmentary portion of an electrical schematic diagram forthe mixer control circuit of the present invnetion; and

FIG. 5 is an enlarged, fragmentary elevational view looking in thedirection of arrows 5-5 of FIG. 2.

Referring now more particularly to the drawings, therein is illustrateda new and improved multiple stage, continuous mixing apparatus, referredto generally by the reference numeral 10, and constructed in accordancewith the features of the present invention. The apparatus 10 isgenerally similar to the apparatus shown in the aforementioned patent,which patent is owned by the same assignee as the present invention. Themixer 10 is adapted to mix and mull various types of bulk particulatematerials including, for example, foundry sand on a continuous ratherthan a batch basis. The mixer is adapted to be fed from a feed beltconveyor 12 or other suitable conveyance for supplying material to themixer at the desired flow rate and the belt conveyor is driven by anelectric gear motor 14 or other suitable motive power source. The mixer10 includes a pair of generally cylindrical mixing chambers 16 and 18which may be appropriately designated as the inlet or charging chamberand the outlet or discharging chamber. The chambers are in directcommunication with one another in order to accommodate the transfer ofmaterials therebetween with a net flow from the charge chamber 16 to theoutlet chamber 18. As best shown in FIGS. 1 and 2, each mixing chamberincludes an upstanding vertical side wall portion 20 supported on acircular bed or base plate 22 and in plan view the bed plates of therespective chambers intersect one another at chordal segments to form agenerally FIG. 8 shaped silhouette when viewed from the top as in FIGS.1 and 3. The bed plate structure of the chambers is supported from thefloor or other supporting structure by a plurality of vertical supportlegs 24.

In each mixing chamber 16 and 18 there is provided a mixing headassembly generally indicated by the reference numeral 26 and rotatableabout an upstanding vertical center axis at the center of each circularbed plate 22. As described in the aforementioned patent, each mixinghead assembly 26 includes a pair of large, heavy, floating mullingwheels 28 mounted on support axle structures extending from oppositesides of the central axis of rotation of the mixing head. These mullingwheels are mounted for free rotation on suitable support axles and theouter periphery of the wheels directly engage the material in the mixingchamber for kneading and mulling the material as the mixing head rotatesto break up lumps therein. Each mixing head assembly 26 includes aturret head 30 mounted at the upper end of a vertical drive shaft 32,which shaft is supported at its lower end by an annular bearing assembly34 carried at the center of the circular bed plate 22 of its mixingchamber. The mixing heads include an outwardly extending arm 36 forsupporting an outer plow assembly 38 adapted to move in a circular patharound the outer periphery of the mixing chamber as the mixing headsrotate. As indicated in FIGS. 1 and 3 the mixing head assembly in thecharging and discharging mixing chambers 16 and 18 are driven to rotatein opposite directions as indicated by the arrows A and B and the outerplow assemblies 38 traverse or pass over a common area of ellipticalshape (indicated by cross hatching and the letter C) adjacent thejunction between the cylindrical mixing chambers. Each mixing head 26also includes an inner plow assembly 40 supported on the turret head 30on a side opposite the outer plow assembly 38. The inner plows areadapted to move material outwardly from the central portion of themixing chambers into the path traversed by the mulling wheel 28.

As best indicated in FIGS. 1 and 3 material is delivered into thecharging chamber 16 of the mixer from a feed conveyor 12 and is thenmoved around the chamber in a generally counterclockwise direction (asindicated by the arrows A") while being mixed and mulled by the plowassemblies and the mulling wheels 28. Material from the inlet chamber ispicked up in the common area between chambers (designated C) by theouter plow assembly 38 of the mixing head 26 in the outlet or dischargechamber 18. The mixing head in the discharge chamber 18 rotates in anopposite or clockwise direction (as indicated by the arrows B). Theouter plow assemblies 38 of both mixing heads are adapted to mix andmove the material around the outer periphery of the mixing chambers anddeflect the material inwardly toward the center into the path of themulling wheels 28 and the inner plow assemblies 40 are adapted to mixand move the material around the central portion of the chambersoutwardly into the path of the mulling wheels.

In accordance with the invention the mixing; heads 26 in the respectiveinlet and outlet mixing chambers 16 and 18 are driven in selected phaserelation with respect to one another to provide a net flow of materialfrom the inlet to the outlet chambers and to prevent the outer plowassemblies 38 from colliding or engaging one another in the common areaC. In addition, the normal phase relation between the respectiverotative mixing heads 26 in the chambers 16 and 18 is selected so thattransfer of material back and forth between the chambers occurs more orless continuously. A net transfer of material from the inlet chamber 16to the outlet chamber 18 is obtained because the outer plow assemblies38 of the respective mixing heads although driven to rotate at the samespeed are not 180 out of phase. In other words the outer plows 38 of therespective mixing heads do not follow one another across the common areaC by equal time intervals during each revolution. Material received inthe chamber 16 is eventually transferred to the chamber 18 by the rotarymixing heads 26 and is then discharged from the outlet chamber 18through a discharge opening in the wall 20 which is controlled by amovable discharge door 42. The door is mounted on a shaft 44 supportedfrom opposite sides of a discharge chute 46 and the opening and closingof the door is controlled by an air cylinder 48. The material dischargecontrol system of the mixer 10 is preferably of the type shown anddescribed in U.S. Pat. No. 3,395,834 which patent is incorporated hereinby reference.

Each of the mixing heads 26 includes a drive shaft 32 coupled to theoutput shaft of a reduction gear box assembly 50 secured to theunderside of the bed plate structure 22 at the center of the mixingchamber. The output shaft of the reduction gear boxes are directlycoupled to their respective drive shafts by means of cooperating annularflange coupling members 52 and 54 which are carried on the ends of therespective shafts. These flange couplings are drivingly connected by oneor more shear pins 56 which are of a selected strength and adapted toshear upon prescribed maximum levels of drive torque between the reducerand mixing head. The shear pins 56 provide a safety system fordecoupling the output shaft of the gear box from the mixing heads whenover torquing occurs. If for example, an excessive volume of material isfed to rapidly discharge into the inlet mixing chamber 16 causingexcessive torque and force on the mixing head 26 to develop, the shearpin 56 will fail and thereby decouple the mixing head from the outputshaft of the gear box to prevent damage to the system and furtheroverloading of the drive motor. Each reduction gear box assembly 50 issupplied with rotative power from a common, interconnecting drive shaft58 (FIG. 1). Opposite ends of the shaft are connected to the inputshafts on the respective gear boxes 50 through couplings 60. Thecouplings 60 may be of the shear type designed with a shear pin to failand provide decoupling when a maximum torque level is reached. The shaft58 is driven via a belt or chain drive system 62 which includes aplurality of endless chains or belts 64 entrained around a drive pulley66 and a driven pulley 68 mounted on the shaft 58. The drive pulley 66is mounted on a motor extension shaft 70 supported in a pair of bearings72 mounted on a power unit base 74. The shaft 70 is coupled to a sourceof rotative power output such as the rotor shaft of an electric motor'76 having suitable horsepower, speed and torque characteristics.Coupling is achieved by means of an electrically actuated clutch 78operated by compressed air and controlled by a solenoid type clutchcontrol air valve 80. The clutch air valve 80 is adapted to selectivelysupply or exhaust compressed air at suitable operating pressure into orout of the clutch via a passage in the motor shaft. When the solenoid isenergized, compressed air is supplied via the clutch valve 80 to engagethe clutch and the air is supplied via an inlet conduit 82 connected toa suitable source of compressed air. When the solenoid of the clutch airvalve 80 is deenergized, the clutch is disengaged by venting the fluidto the atmosphere and the electric motor 76 is decoupled from the outputshaft 70 so that rotative power is no longer provided for the mixingheads 26.

From the foregoing, it will be seen that the source of rotative powersuch as the motor '76 may be decoupled from the mixing heads in a numberof different ways including the de-energization of the motor itself,deenergizing the clutch control valve 80, shearing of the pins 56 of theindividual mixing head drive shafts and shearing or failure of othercomponents in the power train from the motor to the mixing heads.

in accordance with a feature of the present invention, a pair of sensingdevices 100 (FIG. 5) are provided for monitoring the selected phaserelationship between the mixing head assemblies 26 and for detecting anychange from the desired phase relationship between the heads. Eachsensing device 100 includes a proximity switch 100 which is activatedonce during each revolution of its respective mixing head assembly 26and in the illustrated embodiment, the proximity switches 102 comprisemagnetic reed switches having a pair of relatively movable, magneticreed contacts 104 which open and close to make and break contact. Theswitches close when a magnetic means such as a permanent magnet disc 106is passed directly below the reed contacts 104 and the switches openwhen the disc moves away. The magnetic discs 106 are mounted on uprightsupport ports 108 carried on the respective mixing heads 26 and eachsensing device 100 further includes one or more electrical leads 110 forconnecting the reed switches 102 with a suitable electrical controlcircuit. The magnetic discs 106 and support posts 108 are positionedsimilarly on each of the rotatable mixing head assemblies 26, forexample, on the lateral arms 36 supporting the outer plows 38. Theproximity switches 102 are thus actuated as the arms pass thereby onceduring each revolution of the mixing head assemblies 26. The proximityswitches 102 are fixedly mounted on support structures 112 and areadjusted such that a very small clearance x is provided upon the passageof the magnetic disc 106 below the proximity switches 102.

In accordance with a feature of the invention, one sensing device 100 isprovided for each of the chambers 16 and 18 which contain a rotatingmixing head 26 and the devices are positioned so that both proximityswitches 102 are activated or closed at exactly the same moment in timeby the respective magnetic discs 106. An optimum position for mountingthe sensing devices 100 to support their proximity switches 102 forsimultaneous actuation is illustrated in FIGS. 1 and 3 although therelative positioning of the sensing devices 100 may be changed to otherpoints on the perimeters of the mixing chambers. The position chosen forfixedly mounting the sensing device 100 for the chamber 16 must becoordinated with the position chosen for fixedly mounting the othersensing device 100 for the chamber 18 so that two proximity switches 102will be activated at the same moment in time.

In accordance with a feature of the present invention, a control panel120 (FIG. 3) is provided to enable the mixing apparatus to beautomatically or manually controlled. The mixer and control panel 120 issupplied with power by input leads 122 carrying high voltage,three-phase, alternating current for operating the drive motor 76 andthe motor 14 for the feed conveyor 12. Input leads 124 supply relativelylow voltage, singlephase, alternating current power for operating thestarter for the mixer motor and the feed belt conveyor drive motor 14and the solenoid controlled, clutch air valve 80.

A main circuit disconnect 126 is provided for removing all control powerto the mixing apparatus 10. The mixing apparatus 10 is energized byplacing a control power switch 128 in the on" position and subsequently,the drive motor 76 my be energized by pressing a start button 130. Thedrive motor may be stopped at any time by pressing a stop" button 132.

After the drive motor 76 has been started, the mixing head assemblies 26may be actuated to begin their rotation by energizing the solenoidcontrolled, clutch air valve 80. The solenoid air valve 80 may beenergized manually by depressing a start button 134 or deenergized atany time by depressing a stop button 136. When the solenoid air valve 80is energized, compressed air is supplied to the clutch 78 to drivinglyinterconnect the motor extension shaft and the output shaft of the motor76. When the solenoid air valve is de-energized, the compressed air fromthe clutch is vented to the atmosphere to thereby disengage the clutch78. When the motor extension shaft 70 is rotated both mixing headassemblies 26 begin to rotate as previously described. After the mixinghead assemblies 26 are rotating, the feed belt conveyor drive motor 14is energized by depressing a start button 138 to feed material into themixer. The feed belt conveyor drive motor 14 may be de-energized at anytime by depressing a stop button 140 to stop the flow of material to themixer.

A series of relay interlocks are provided so that the feed belt conveyordrive motor 14, which drives the feed belt conveyor 12, may not beenergized until the mixing head assemblies 26 are rotating and otherrelay interlocks are provided to insure that the clutch is not engagedunless the drive motor 76 is running.

A phase failure light 142 is provided on the control panel 120 (FIGS. 3and 4) to indicate visually an alarm whenever the mixing head assemblies26 are sensed to be out of the proper phase relationship and when thisoccurs, the mixer 10 is automatically shut down and further operation isprevented until a reset button 144 is depressed. A control circuitincorporating a pair of sensing devices 100 for the chambers 16 and 18for use in monitoring the selected phase relation of the two mixing headassemblies 26 is illustrated in FIG. 4. The low voltage, single-phase,alternating current power from the input lines 124 (FIG. 3) is suppliedto a pair of terminals 162 (FIG. 4) with fuses 164 provided forinterrupting the power flow if an overload condition occurs.

The control circuit 160 includes a time delay relay 166 having anadjustable preset time delay characteristic or capability which is setup to provide a timed interval slightly greater than the time requiredfor a mixing head assembly 26 to complete one full revolution when it isoperating on speed. When the time delay relay 166 is de-energized, itbegins a time out interval and at the end or timing out of the period, arelay contact 166a controlled thereby is closed. The timing period ofthe time delay relay 166 can be interrupted and the relay is reset whenalternating current power is again resupplied to the relay 166 beforethe completion of the original timing period. Initially, alternatingcurrent power is supplied to the timing relay 166 (FIG. 4) through anormally-closed, clutch control, relay switch 80a operatively associatedwith the solenoid air valve 80 for controlling the mixer clutch '78.When the start button 134 is depressed to engage the clutch 78, therelay switch 80a is opened to activate the phase sensor system and anormally-open clutch control relay switch stlb is closed to ready theFAILURE ALARM and STOP ClRCUlTS. When this occurs, the time delay relay166 is de-energized and begins its timing period (which period isadjustably preset for an interval slightly greater than the timerequired for the mixing head assemblies 26 to complete one fullrevolution when on-speed). in normal operation, with on-speed and properphase relation between the heads 26, the time-out period of the relay166 is interrupted by the simultaneous activation and closure of theproximity switches 102 which action momentarily energizes or resets therelay and starts a new timing period before the prior timing period hadexpired.

Various occurrences may result in the time-out period of the relay 166failing to be RE-ACTIVATED or reset before completion. For example, oneor more of the belts or chains 64 of the belt drive assembly 62 may beslipping and cause the mixing head assemblies 26 to rotate at a slowerspeed than normal and this condition may permit the relay 166 to fullytime-out. On start up of the mixer, the time required after initialengagement of the clutch 78 until normal on-speed operation of themixing heads is achieved may be too long indicating belt or clutchslippage and this will permit the relay to time-out. One of the shearpins 56 or a shear pin associated with one of the couplings 60 may havesheared because of excessive torque due to an obstruction resulting inthe stoppage or slowage of rotation of one of the mixing head assemblies26 while the other continues at normal speed. When this occurs, only oneof the sensor switches 102 is activated at one time because of the outof phase relation and the time delay relay is permitted to time out. Thetwo mixing head assemblies 26 may have been initially set up with animproper phase relationship or after maintenance the drive trains maynot have been reconnected with the proper phase relationship between themixing heads and the proximity switches 102 may have been improperlypositioned. If one of the above occurrences takes place, both of theproximity switches 162 (FIG. 4) will fail to close simultaneously,thereby permitting the time-out period of the relay 366 to expire.

At the expiration of a complete time-out period of the relay 166, theTIMED CONTACT 166a closes and energizes a failure relay 168. Energizingof the failure relay 168 closes two pairs of normally-open relayswitches 168a and 16822 and opens one or more pairs of normally-closedswitches l68c, 168d, 168s etc. The reset button 244 is normally closed,and the failure relay 168 is then held energized through the now closedswitch 168a. Closure of the switch 168b illuminates the phase failurelight 142 and if desired, an external, audible, visual or other type ofalarm may be connected between a pair of terminals 170 to indicate aphase failure.

The relay switch 168a is wired in series in the lines supplying power tothe solenoid controlled, clutch operating air valve 80 and the switch168d is in series in the power lines to the starter of the feed beltconveyor drive motor 14. Opening of the switches 168s and 168d, causesthe de-energization of the clutch control valve 80 and deactivates thefeed belt conveyor drive motor 14 stopping the mixer and the flow ofmaterial supplied thereto. The failure relay 168 is maintained in anenergized condition via the reset button 144 and the closed holdingrelay contacts 168a, and it is not possible to re-energize the clutchcontrol solenoid air valve or the feed belt conveyor drive motor 14until the reset button 144 is depressed. Thus a positive physical actionby an operator is required before the mixing apparatus 10 may be placedin an operative mixing head condition after a phase failure hasoccurred.

Many modifications and variations of the present invention are possiblein light of the above teachings. it is therefore to be understood that,within the scope of the appended claims, the present invention may bepracticed otherwise than as specifically described.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:

1. In a multiple stage mixer, a pair of mixing chambers in communicationwith each other for the transfer of material therebetween, a rotarymixing element in each chamber movable on a path overlapping into theother chamber and in selected normal phase relation with the mixingelement therein, a source of motive power for rotatively driving saidmixing elements, and control means for rendering said motive powersource ineffective to rotatively drive said mixing elements when saidelements are out of said normal phase relation with each other.

2. The mixer of claim 1 wherein said control means includes a pair ofsensing devices activated by the relative position of said mixingelements in their respective mixing chambers.

3. The mixer of claim 2 wherein said motive power source includes motormeans, a drive shaft drivingly connected with said mixing elements, andclutch means for selectively coupling and decoupling said motor meansand said shaft, and means controlled by said sensing devices fordecoupling said clutch means when said elements are sensed to be out ofsaid normal phase relation.

4. The mixer of claim 3 wherein said sensing devices comprise a pair ofswitches interconnected in series and positioned on said mixing chambersfor simultaneous activation by the rotation of said rotary mixingelements in proper phase relation with respect to each other.

5. The mixer of claim 4 wherein said control means includes a timeractivated to begin an adjustable timing cycle by the simultaneousactivation of said sensing switches.

6. The mixer of claim 5 wherein said control means includes circuitmeans for controling said clutch means to decouple said motor means andsaid shaft when simultaneous activation of said sensing switches doesnot occur within a selected time interval set up on said timer.

7. The mixer of claim 6 wherein said control means includes alarm signalmeans for alerting an operator whenever said sensing switches are notactivated simultaneously during each revolution of said mixing elements.

8. In combination, a multiple stage mixer including a pair of mixingchambers in communication with each other for the transfer of materialtherebetween, a material feeder for delivery of material to at least oneof said chambers for mixing therein, a rotary mixing element in eachchamber movable on a path overlapping into the other chamber and inselected normal phase relation with the mixing element therein, a sourceof motive power for rotatively driving said mixing elements, and controlmeans for rendering said motive power source ineffective to rotativelydrive said mixing elements when said elements are out of said normalphase relation with each other, said control means including means forpreventing said feeder from feeding material into said chamber when saidmixing elements are out of phase.

9. The combination of claim 8 wherein said control means includes aclutch for interconnecting said source of motive power and said mixingelements, a pair of sensor switches mounted for simultaneous activationby the in-phase rotation of said mixing elements in said mixingchambers, a timer activated by said simultaneous activation of saidsensor switch to begin an adjustable timing cycle and means activated bythe com pletion of a timing cycle for operating said clutch todisconnect said source of motive power from said mixing elements.

10. The combination of claim 9 wherein said control means includescircuit means for recycling said timer to begin another timing cycleprior to the time outcompletion of the previous cycle when said sensorswitches are activated simultaneously by on-speed rotation of saidmixing elements.

1. In a multiple stage mixer, a pair of mixing chambers in communicationwith each other for the transfer of material therebetween, a rotarymixing element in each chamber movable on a path overlapping into theother chamber and in selected normal phase relation with the mixingelement therein, a source of motive power for rotatively driving saidmixing elements, and control means for rendering said motive powersource ineffective to rotatively drive said mixing elements when saidelements are out of said normal phase relation with each other.
 2. Themixer of claim 1 wherein said control means includes a pair of sensingdevices activated by the relative position of said mixing elements intheir respective mixing chambers.
 3. The mixer of claim 2 wherein saidmotive power source includes motor means, a drive shaft drivinglyconnected with said mixing elements, and clutch means for selectivelycoupling and decoupling said motor means and said shaft, and meanscontrolled by said sensing devices for decoupling said clutch means whensaid elements are sensed to be out of said normal phase relation.
 4. Themixer of claim 3 wherein said sensing devices comprise a pair ofswitches interconnected in series and positioned on said mixing chambersfor simultaneous activation by the rotation of said rotary mixingelements in proper phase relation with respect to each other.
 5. Themixer of claim 4 wherein said control means includes a timer activatedto begin an adjustable timing cycle by the simultaneous activation ofsaid sensing switches.
 6. The mixer of claim 5 wherein said controlmeans includes circuit means for controling said clutch means todecouple said motor means and saId shaft when simultaneous activation ofsaid sensing switches does not occur within a selected time interval setup on said timer.
 7. The mixer of claim 6 wherein said control meansincludes alarm signal means for alerting an operator whenever saidsensing switches are not activated simultaneously during each revolutionof said mixing elements.
 8. In combination, a multiple stage mixerincluding a pair of mixing chambers in communication with each other forthe transfer of material therebetween, a material feeder for delivery ofmaterial to at least one of said chambers for mixing therein, a rotarymixing element in each chamber movable on a path overlapping into theother chamber and in selected normal phase relation with the mixingelement therein, a source of motive power for rotatively driving saidmixing elements, and control means for rendering said motive powersource ineffective to rotatively drive said mixing elements when saidelements are out of said normal phase relation with each other, saidcontrol means including means for preventing said feeder from feedingmaterial into said chamber when said mixing elements are out of phase.9. The combination of claim 8 wherein said control means includes aclutch for interconnecting said source of motive power and said mixingelements, a pair of sensor switches mounted for simultaneous activationby the in-phase rotation of said mixing elements in said mixingchambers, a timer activated by said simultaneous activation of saidsensor switch to begin an adjustable timing cycle and means activated bythe completion of a timing cycle for operating said clutch to disconnectsaid source of motive power from said mixing elements.
 10. Thecombination of claim 9 wherein said control means includes circuit meansfor recycling said timer to begin another timing cycle prior to the timeout completion of the previous cycle when said sensor switches areactivated simultaneously by on-speed rotation of said mixing elements.